The present invention relates to a liquid-crystal panel for use in a liquid-crystal display apparatus, an optical shutter, or the like, to a method of manufacturing the same, and to a liquid-crystal display apparatus using the liquid-crystal panel.
Liquid-crystal panels, which are small in size, light in weight, and operable with low power voltage, have a wide range of applications as means for displaying a variety of visually recognizable patterns. For example, such appliances as wrist watches, electronic tabletop calculators, personal computers, and personal word processors often use liquid-crystal panels therein for their further miniaturization and weight reduction. However, since a liquid-crystal panel of TN (twisted nematic) type or STN (supertwisted nematic) type currently used requires a polarizing plate, its light transmittance and long-term light resistance are unsatisfactory. By contrast, since a panel containing a polymer dispersed liquid crystal (PDLC) does not require a polarizing plate, it is free from the above-mentioned drawbacks.
FIGS. 5(a) and 5(b) are cross-sectional views showing the structure of a conventional liquid-crystal panel of polymer dispersed type. FIG. 5(a) shows the state without a voltage applied, while FIG. 5(b) shows the state with the voltage applied. As shown in FIGS. 5(a) and 5(b), the liquid-crystal panel is constituted by: a pair of glass substrates 1a and 1b having electrodes 2a and 2b, respectively, which are opposed to each other; and a mixture of a liquid crystal 4 and a photo-curing polymer 5 which is filled into the space between the both substrates 1a and 1b. It is also possible to use a thermosetting polymer instead of the photo-curing polymer 5, provided that it contains a crosslinkage-type polymer as its main component.
In a liquid-crystal panel of polymer dispersed type as described above, the liquid-crystal 4 is dispersed and held in the photo-curing polymer 5 or the photo-curing polymer 5 exists in particles or in network in the liquid crystal 4. If a voltage is not applied to the space between the electrodes 2a and 2b, liquid-crystal molecules are randomly oriented so that a difference in refractive index is caused between the photo-curing polymer 5 and the liquid crystal 4, thereby scattering light (see FIG. 5(a)). On the other hand, if the voltage is applied to the space between the electrodes 2a and 2b, the liquid-crystal molecules are oriented in the direction of a resulting electric field so that there is a match in refractive index between the liquid crystal 4 and the photo-curing polymer 5, thereby transmitting light (see FIG. 5(b)). Thus, the liquid-crystal panel of polymer dispersed type utilizes the phenomena of light scattering and light transmission. Spacers dispersed between the substrates 1a and 1b in order to maintain a specified distance therebetween are composed of an insulating material in spherical particles. Specifically, spacers made of a transparent material such as SiO2, benzoguanamine resin, or melamine resin are used at present.
However, the above conventional liquid-crystal panel is disadvantageous in that, when a voltage is not applied to the liquid-crystal panel, i.e., in the black mode, light scattering becomes unsatisfactory in the state shown in FIG. 5(a), so that a sufficient contrast cannot be obtained. Even when the voltage is applied to the liquid-crystal panel, local variations are observed in light transmittance, resulting in uneven display of images.
In view of the foregoing, the present inventors have examined the phenomenon closely, which led to the following findings.
(1) The spacers composed of a transparent polymer material presents no problem if they are used in the liquid-crystal panel of TN type or STN type. However, if they are used in the liquid-crystal panel of polymer dispersed type which utilizes the phenomena of light scattering and light transmission, light scattering becomes unsatisfactory when a voltage is not applied, i.e., in the black mode, so that a sufficient contrast cannot be obtained.
(2) The use of spacers composed of an opaque colored polymer material, instead of a transparent material, is also under consideration as disclosed in Japanese Laid-Open Patent Publication Nos. 63-157130, 1-144021, and 4-15623. Even with the spacers composed of a colored material, however, it is difficult to obtain a sufficient contrast.
(3) When a phase separation is caused by irradiating the photo-curing polymer, which is a crosslinkage-type polymer, with light or by heating the thermosetting polymer, liquid-crystal droplets adjacent to the spacers become larger in size than those not adjacent to the spacers. As a result, the application of a voltage causes a difference in light transmittance between the regions adjacent to the spacers and the other regions, so that a uniform displaying property may not be obtained in a liquid-crystal display apparatus.
The present invention has been achieved in view of the foregoing and has the following objects.
The first object of the present invention is to provide a liquid-crystal panel in which a difference in light transmittance between the state with a voltage applied and the state without the voltage applied is significant, i.e., a high-contrast liquid-crystal panel by using liquid-crystal droplets and spacers with improved structures.
The second object of the present invention is to provide a liquid-crystal panel in which local variations in light transmittance in the state with the voltage applied are reduced by using spacers with an improved structure.
The third object of the present invention is to provide a liquid-crystal display apparatus with an excellent displaying property by using a liquid-crystal panel with excellent characteristics as described above.
To attain the above first object, a liquid-crystal panel with a first structure comprises: a pair of substrates disposed so as to face each other, each being provided with an electrode for applying a voltage; spacers dispersed in the space between the above pair of substrates, the above polymer member not containing a crosslinkage-type polymer; and droplets of a liquid crystal mixed with the above polymer member and filled in the space between the above pair of substrates, the above droplets of the liquid crystal having a mean diameter of 3.0 xcexcm or less.
With the above first basic structure, since the liquid crystal to be filled in the space between the substrates in the liquid-crystal panel is in fine droplets, the function of scattering light when a voltage is not applied is enhanced, so that light leakage via the droplets of the liquid crystal seldom occurs. Consequently, a high contrast can be obtained.
The above spacers can be provided with the function of preventing the transmission of visible light. For this purpose, the spacers may be colored or the surface roughness of the spacers may be increased to a degree sufficient to scatter the visible light.
With the structure, light leakage via the spacers seldom occurs in the black mode without the voltage applied, so that a higher contrast can be obtained.
To attain the above first object, a first method of manufacturing a liquid-crystal panel comprises the steps of: preparing in advance a pair of substrates each being provided with an electrode for applying a voltage and dispersing, on one of the above pair of substrates, spacers for holding the substrates at a specified distance; holding the above substrate on which the spacers are dispersed and the other substrate so that they are opposed to each other with the spacers interposed therebetween, mixing a polymer member not containing a crosslinkage-type polymer with a liquid crystal, and filling the mixture into the space between the substrates; and promoting the crosslinkage of the above polymer member, while causing a phase separation between the above polymer member and the above liquid crystal simultaneously, so as to separate the above liquid crystal in droplets having a mean diameter of 3.0 xcexcm or less. If a photo-curing polymer is used as a material for composing the above polymer member, in particular, the above polymer member is irradiated with light equal to or intenser than 500 mJ/cm2 in the step of promoting crosslinkage of the above polymer member.
In the method, since the dose of light in the step of curing the photo-curing polymer composing the polymer member has been set large, the cross-linking reaction of the polymer member is promoted, so that a distinct phase separation occurs between the liquid crystal and the polymer member. Consequently, the droplets of the liquid crystal, which have been mixed with the polymer member, are miniaturized.
To attain the above second object, a liquid-crystal panel with a second structure comprises: a pair of substrates disposed so as to face each other, each being provided with an electrode for applying a voltage; a polymer member filled in the space between the above pair of substrates, the above polymer member not containing a crosslinkage-type polymer; and spacers dispersed in the space between the above pair of substrates so as to hold the above pair of substrates at a specified distance, the surface portions of the above spacers having at least either of a scare affinity with the droplets of the liquid crystal and a large affinity with the above polymer member.
With the second structure, the effect of the liquid crystal moving far away from the surfaces of the spacers becomes larger than the effect of the liquid crystal moving closer to the surfaces of the spacers. Consequently, during the phase separation caused by crosslinkage of the polymer material, the diameter of the droplet of the liquid crystal in a region adjacent to the spacer does not become larger than the diameter of the droplet of the liquid crystal in a region not adjacent to the spacer. Accordingly, local variations in light transmittance of the liquid-crystal panel in the state with the voltage applied are minimized.
In a specific structure of the above liquid-crystal panel with the second structure, the surface portions of the spacers may be treated so as to have the property of rejecting the liquid crystal. Consequently, the liquid crystal is kept away from the surfaces of the spacers in the liquid-crystal panel, so that the diameter of the droplet of the liquid crystal does not become particularly large in a region adjacent to the spacer.
In another specific structure, at least the surface portions of the spacers are composed of a polymer material of the same type as that of the above polymer member, with the result that the polymer member preferentially approaches the surfaces of the spacers. Consequently, the diameter of the droplet of the liquid crystal is not increased in a region adjacent to the spacer.
It is also possible to add the above second structure to the liquid-crystal panel with the above first structure.
In the liquid-crystal panel with the above first or second structure, a photo-curing polymer can be used as the crosslinkage-type polymer serving as the main component of the above polymer member.
With the structure, a phase separation is caused between the polymer member and the liquid crystal by utilizing the cross-linking property of the photo-curing polymer, so that an excellent dispersibility is imparted to the droplets of the liquid crystal.
A second method of manufacturing a liquid-crystal panel comprises the steps of: preparing in advance a pair of substrates each being provided with an electrode for applying a voltage and dispersing, on one of the above pair of substrates; spacers for holding the substrates at a specified distance, the surface portions of the above spacers having either a scarce affinity with droplets of a liquid crystal and a large affinity with a polymer member; holding the above substrate on which the spacers are dispersed and the other substrate so that they are opposed to each other with the spacers interposed therebetween, mixing the above polymer member not containing a crosslinkage-type polymer with a liquid crystal, and filling the mixture into the space between the substrates; and promoting the crosslinkage of the above polymer member, while causing a phase separation between the above polymer member and the above liquid crystal simultaneously.
Specifically in the above second method of manufacturing a liquid-crystal panel, a surface-active agent containing fluorine as its main component is adsorbed by the surfaces of the spacers so that the surface portions of the spacers have the property of rejecting the above liquid crystal or the spacers at least the surface portions of which are composed of a material of the same type as that of the above polymer member are used.
With the above method, the liquid-crystal panel with the second structure can be obtained.
To attain the above third object, the liquid-crystal display apparatus of the present invention comprises in addition to the liquid-crystal panel with the above first or second structure: light radiating means for irradiating the liquid-crystal panel with light; a driving circuit for applying an electric signal to the above liquid-crystal panel; and a displaying means for displaying the pattern of light outputted from the above liquid-crystal panel.